Generally, in the preparation of plastic films from granular or pelleted polymer resin, the polymer is first extruded to provide a stream of polymer melt, and then the extruded polymer is subjected to the film-making process. Film-making typically involves a number of discrete procedural stages, including melt film formation, quenching, and windup.
An optional part of the film-making process is a procedure known as “orientation.” The “orientation” of a polymer is a reference to its molecular organization, i.e., the orientation of molecules relative to each other. Similarly, the process of “orientation” is the process by which directionality (orientation) is imposed upon the polymeric arrangements in the film. The process of orientation is employed to impart desirable properties to films, including making cast films tougher (higher tensile properties). Depending on whether the film is made by casting as a flat film or by blowing as a tubular film, the orientation process requires substantially different procedures. This is related to the different physical characteristics possessed by films made by the two conventional film-making processes: casting and blowing. Generally, blown films tend to have greater stiffness, toughness and barrier properties. By contrast, cast films usually have the advantages of greater film clarity and uniformity of thickness and flatness, generally permitting use of a wider range of polymers and producing a higher quality film.
Orientation is accomplished by heating a polymer to a temperature at or above its glass-transition temperature (Tg) but below its crystalline melting point (Tm), and then stretching the film quickly. On cooling, the molecular alignment imposed by the stretching competes favorably with crystallization and the drawn polymer molecules condense into a crystalline network with crystalline domains (crystallites) aligned in the direction of the drawing force. As a general rule, the degree of orientation is proportional to the amount of stretch, and inversely related to the temperature at which the stretching is performed. For example, if a base material is stretched to twice its original length (2:1) at a higher temperature, the orientation in the resulting film will tend to be less than that in another film stretched 2:1 but at a lower temperature. Moreover, higher orientation also generally correlates with a higher modulus, i.e., measurably higher stiffness and strength.
When a film has been stretched in a single direction (monoaxial orientation), the resulting film exhibits great strength and stiffness along the direction of stretch, but it is weak in the other direction, i.e., across the stretch, often splitting or tearing into fibers (fibrillating) when flexed or pulled. To overcome this limitation, two-way or biaxial orientation is employed to more evenly distribute the strength of the film in two directions, in which the crystallites are sheet like rather than fibrillar. These biaxially oriented films tend to be stiffer and stronger, and also exhibit much better resistance to flexing or folding forces, leading to their greater utility in packaging applications.
From a practical perspective, it is possible, but technically and mechanically quite difficult, to biaxially orient films by simultaneously stretching the film in two directions. Apparatus for this purpose is known, but tends to be expensive to employ. As a result, most biaxial orientation processes use apparatus which stretches the film sequentially, first in one direction and then in the other. Again for practical reasons, typical orienting apparatus stretches the film first in the direction of the film travel, i.e., in the longitudinal or “machine direction” (MD), and then in the direction perpendicular to the machine direction, i.e., the “cross direction” (CD).
The degree to which a film can be oriented is also dependent upon the polymer from which it is made. Polypropylene, as well as polyethylene terephthalate (PET), and NYLON, are polymers which are highly crystalline and are readily heat stabilized to form dimensionally stable films. In the plastics industry, common biaxial oriented or “biax” films include MYLAR (biaxial oriented polyester or BOPET), NYLON (biaxial oriented polyamide or BOPA), and biaxial oriented polypropylene (BOPP). Biaxial oriented polymeric films and methods of fabricating them are known in the art and are described, for example, in U.S. Pat. No. 6,379,605 to Lin, U.S. Pat. No. 6,174,655 to Shirokura, et al., U.S. Pat. No. 5,912,060 to Kishida, et al., U.S. Pat. No. 5,552,011 to Lin, and U.S. Pat. No. 5,268,135 to Sasaki et al., which are incorporated herein by reference.
On-line measurements of the thickness, basis weight, and molecular orientation of plastic films can be employed to control the process of fabricating biaxial oriented plastics. Orientation within a film can be described by the index of refraction ellipsoid, which is defined by the indices of refraction along the three axial directions, i.e., machine direction, cross-direction and thickness. Birefringence is the difference between two of these refraction indices. In thin films, birefringence of particular interest is the in-plane birefringence, which is defined as the difference between the indices of refraction along the machine direction (MD) and the cross-direction (CD). Birefringence in polymers is a result of the anisotropy in the molecular orientation. Such anisotropy occurs in the biax fabrication process wherein stretching of the film leads to molecular orientation in the machine and cross directions.
An optical technique for determining thickness measures the amounts of light absorbed by a sample in two or more wavelength bands of the infrared (IR) spectrum. In the simplest case, two bands are used, a measure band and a reference band. The measure band is selected to coincide with a strong absorption in the target material (film to be measured), and the reference band is selected to match a weakly absorbing region of the target material.
The transmission measurement is based on Beer's Law, which states I=I0e−μw, where I0 is the signal with no sample, I is the signal with sample, μ is the absorption coefficient, and w is the weight of the sample. Equivalently, this may be written as w=(1/μ) ln(I0/I). Thus for a given wavelength of IR radiation, the weight, or thickness of the film, is proportional to the logarithm of the attenuation.
In practice the accuracy of such transmission techniques is limited when measuring in the thin film regime due to an interference fringing effect. Fringes in the transmission and reflection spectra of the measured film appear due to interference of the light reflected from the film surfaces with light transmitted through the film. An example is illustrated in FIG. 1, which shows interference fringes 31 forming when the transmission of a 16 μm polyamide film is measured at different wavelengths. As a result, the sensor calibration error for such films increases significantly making measurements inaccurate. The lower limit for the film thickness is about 15-30 microns and depends on the material of the film.
To understand the fringing effect, consider a thin film with thickness d and index of refraction n2, deposited on another material as shown in FIG. 2. Both the top and bottom of the film will reflect a portion of the light. The total amount of transmitted light contains contributions from these multiple reflections. Because of the wavelike nature of light, the reflections from the two interfaces may add together constructively or destructively, depending on their phase relationship. Their phase relationship is determined by the difference in the optical path lengths of reflections from these two interfaces, which in turn is determined by the thickness of the film d and the index of refraction n. Reflections are in-phase and therefore add constructively when the light path is equal to an integral multiple of the wavelength of light. For light perpendicularly incident on a film, this occurs when 2nd=iλ, where d is the thickness of the film, i is an integer, and λ is the free space wavelength of the incident radiation. Conversely, reflections are out of phase and add destructively when the light path is half of a wavelength different from the in-phase condition, or when 2nd=(i+½)λ.
Qualitatively, these multiple reflections result in a transmission amplitude with a cos(4πnd/λ) component, or a transmitted intensity given by:I=B0+A0cos(4πnd/λ).  (1)
The reflected intensity will have a similar periodic component.
From this it is apparent that the transmittance will vary periodically with wave number 2π/λ. Furthermore, at a given wavelength (index of refraction n is wavelength dependent) the frequency of oscillations is proportional to film thickness d. The transmitted light can be detected by sensors located on the opposite side of the film. A fit of the transmission spectra to Eq. 1 will give the thickness d assuming that n(λ) is known.
Because the spectral position of the fringes depend on the film thickness, there have been efforts to extend current transmission sensors into the thin film regime by measuring interference fringes and extracting the film thickness from the fringe parameters.
In on-line monitoring applications, birefringence is usually obtained directly by measuring the optical retardation using polarimetry techniques. Such a technique is described in U.S. Pat. No. 5,864,403 to Ajji, et al. Retardation is the product of birefringence and thickness of a material. Therefore, it decreases with decreasing birefringence and with decreasing thickness. In the limit of very thin films (below 20-30 μm), retardation is difficult to measure. This is due to the fact that it becomes small and that interference fringes can affect the measurement. The present invention is directed to the use of interference fringes for the measurement of birefringence of thin films.